In certain data communication systems such as V.32 modems, there are two clocks that must be synchronized, known as the near-end and the far-end clocks. The near-end clock is the clock that a given station uses to transmit the data. The far-end clock is the other station's clock. For example, two stations may transmit and receive data according to the V.32 specification. V.32 specifies transmission at 9600 bits per second (bps). However, the V.32 standard allows 100 parts-per-million (ppm) or approximately 0.01% variation in clock frequency. Thus the near-end station may be transmitting data at 9601 bps while the far-end station is transmitting data at 9599 bps. A V.32 modem must be able to synchronize the data without introducing distortion and without costly circuitry.
The modem must also solve the problem of echo. A station's received signal includes not only data from the other station, known as far-end data, but also an echo due to the fact that the interface to the common transmission medium used for both reception and transmission reflects some of the transmit data's signal power back to the station. One type of echo, known as near-end echo, results from the signal power reflected by the near-end station's own interface to the transmission medium. A second type of echo, known as far-end echo, results from the signal power reflected from the far-end station's interface to the medium. The basic scheme used to cancel this echo is to substract a delayed fraction of the transmitted signal from the received signal.
A recent improvement in technology is the development of digital signal processors (DSPs), which are data processors optimized for signal processing operations. DSPs have allowed many V.32 modem functions to be performed by mathematical operations on digital data, and have reduced the number of integrated circuits required to implement a data modem to a minimum. However, known clock synchronization systems require external analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) in addition to the DSP. Thus it is desirable to further reduce the number of integrated circuits required to perform signal processing functions.